The present invention relates to a program-controlled shutter of the type in which an exposure control mechanism comprises a plurality of shutter blades which also serve as the aperture blades and driving means such as a stepping motor for driving the shutter blades. The opening pulse signal to be applied to the driving means for opening the shutter blades so as to stop down the camera lens to a suitable aperture or f-number depending upon the brightness of a subject or light or exposure value VE as well as the closing pulse signal to be applied also to the driving means for closing the shutter blades a suitable exposure time, which is a function of the f-number set for the given exposure value, are generated in response to a digital signal representative of the brightness of the subject.
There have been devised and demonstrated various types of the program-controlled shutters. In one type of the program-control shutters called the triangle opening type, the shutter blades which also function as the aperture blades, are continuously being opened from the completely closed position to the wide opened position, and when a predetermined time is elapsed, the shutter blades are rapidly closed even when they are being still opened toward the wide opened position. In the program-controlled shutters of the type described, the tension spring is generally used to store the energy when the film is advanced, so that the energy may be liberated to open and then return the shutter blades to their initial closed position. In this case, in general, a governor mechanism is used to retard the returning speed of the shutter blades so as to change the shutter speed or exposure time, thereby controlling the exposure over a wide range. As a result, the conventional program-controlled shutters are very complex in construction.
Furthermore, the shutter blade driving force stored in and released from the spring is, in general, not uniform, so that the exposure characteristic or light-flux vs. time characteristic of the shutter changes from time to time. This inherently undesirable feature is much pronounced when the parts of the mechanical shutter are worn out, and the abrasion and wear of the mechanical parts adversely affect the service life of the shutter.
In another type of the program-controlled shutter, the shutter blades are opened until they are stopped by aperture control means whose position in turn, is controlled by a galvanometer in an exposure meter, and the shutter blades remain opened for a desired time. However, the shutter blades are opened only after the aperture control means has been displaced to and securely held in a controlled position in response to the movement of the scanner means which is arrested by the meter needle of the galvanometer. As a consequence, there is some time lag between the time when the brightness of a subject is detected and the time when the shutter blades are actuated because of the time required for the scanner means to engage with the meter needle and the time required for the aperture control means to be displaced to and held in a predetermined control position, so that the shutter operation cannot sufficently respond to relatively rapid changes in brightness of a subject. Furthermore, the program-controlled shutters of the type described have further undesirable features in that the galvanometer is extremely susceptible to damage due to mechanical shocks and that the construction is very complex.